8. THE FUTURE

The future is promising: (1) the census of BHs is expected to grow
rapidly as HST reaches its full potential and as new techniques allow us to
measure
M in
more distant galaxies; (2) the ongoing unification of the
BH and galaxy formation paradigms is fundamental progress, and (3) x-ray
satellites and gravitational wave detectors are expected to probe the
immediate vicinity of the Schwarzschild radius.

Measuring
M by
making dynamical models of observations that
spatially resolve the central kinematics
(Tables 1 and
2) are well tested
techniques. Confidence is growing that the resulting BH masses are
accurate to within ~ 30% in the best cases. This has allowed us to begin
demographic studies of BHs in nearby galaxies. But the above techniques
have a fundamental limitation. They cannot be applied unless the
galaxies are close enough so that we can spatially resolve the region
that is dynamically affected
by the BH. Within a few more years, the most interesting galaxies that are
accessible with HST resolution will have been observed, and new
detections will slow down. Expected advances in spatial resolution will
enable important but only incremental progress. The subject could use a
breakthrough that allows us to measure BH masses in much more distant
objects.

Reverberation mapping exploits the time delays measured between
brightness variations in the AGN continuum and in its broad emission lines.
These are interpreted as the light travel times between the BHs and the
clouds of line-emitting gas. The result is an estimate of the radius
r of the broad-line region. We also have a velocity V from
the FWHM of the emission lines. Together, these measure a mass
MV2r / G. However, a number of authors
(Wandel 1999;
Ho 1999;
Wandel, Peterson, &
Malkan 1999)
have pointed out that reverberation mapping BH masses are systematically
low in the
M -
MB, bulge correlation. Recently, Gebhardt et nuk.
(2000c;
see Figure 6, below, for an update) have shown
that reverberation mapping BH masses agree with the
M -
E
correlation. This suggests that the problem uncovered in previous
comparisons was that the bulge luminosities of the reverberation mapping
galaxies were measured incorrectly or were inflated by young stars.
Gebhardt et nuk.
(2000c)
and Figure 6 here
suggest that reverberation mapping does produce reliable BH masses.

Similarly, ionization model BH masses - ones based on the observed
correlation between quasar luminosity and the radius at which the
broad-line-emitting gas lives - are largely untested and therefore
uncertain.
Laor (1998) and
Gebhardt et nuk.
(2001)
now show that this technique also appears to produce
M
values with no systematic offset from other techniques
(Figure 6).

These results are important because neither reverberation mapping nor
ionization models require us to spatially resolve the central region
affected by
the BH. Both techniques can be applied to objects at arbitrarily large
distances. Therefore BH masses can now be estimated for quasars out to
redshifts of nearly z = 6. Ongoing surveys like 2dF and the Sloan
Digital Sky Survey are producing thousands of quasar detections. BH
masses should therefore be derivable for very large samples that span
all redshifts from z = 0 to the most distant objects known. It
will be important to check as well as possible that the physical
circumstances that make the ionization models
work so well are still valid far away. Nevertheless, it should be possible
to directly measure the growth of BHs in the Universe.